Combustion instability in a composite solid propellant rocket motor is a phenomenon which can be considered to be occurring when the actual chamber pressure observed in the rocket motor exceeds, while operating in the manner intended, the chamber pressure for which the motor was designed, based on propellant burn-rate data obtained from small motor or strand-burning tests. Combustion instability can be characterized either by (a) acoustic oscillations in the chamber pressure which may continue for a portion or all of the burn time of the motor and ususally at a mean pressure higher than that predicted for normal operations or (b) a sudden rapid rise in the chamber pressure which can cause catastrophic failure of the motor. In those cases in which a catastrophic failure of the rocket motor does not occur as a result of the instability, it is still an undesirable phenomenon since most solid propellants are targeted, or their operation predicted, on a knowledge of the pressure-time trace of the rocket motor which directly affects the thrust produced. If the actual chamber pressure is higher than that for which the motor is designed, the motor cannot be used to provide the desired performance with acceptable reliability since predictable ballistic performance depends on a predictable thrust vs. time curve which, in turn, depends on a predictable pressure vs. time program.
Various factors affect combustion instability, and, in general, combustion instability is more likely to occur when rocket motors (a) are operated at higher chamber pressures (b) have greater lengths or length to diameter ratios (c) employ propellants having high solid loadings of oxidizers or (d) are operated at elevated grain temperatures. Various approaches have been taken in the past to solve the problems of combustion instability. These approaches have included the use of resonance rods, the use of exotic grain designs, the use of mass addition techniques to upset the circulation pattern within the combustion chamber, the use of additives such as those disclosed in U.S. Pat. No. 3,336,751, Rifkin et al, Solid Propellant Composition Containing Liquid Organometallic Compound and Method of Use, the use of noncombustible particulate additives, and the addition of a material such as aluminum which produces a fine particulate oxide in the combustion chamber, thereby in some way decreasing the tendency toward combustion stability. The latter technique has been so widely used that state-of-the-art propellants now normally include aluminum as a fuel material, workers in the art sometimes forgetting that the original function of the aluminum was as a stability enhancer.
Unfortunately in many applications, the smoke produced by an aluminum containing propellant cannot be tolerated. For example, in manned spacecraft in which the viewports or cameras are in a position to be exposed to the exhaust fumes, the fogging created upon the firing of such motors can cause complete obscuration of vision. In spin stabilized rocket motors the centrifugal action of the motor causes erratic burning as a result of the centrifuging of the aluminum oxide particles to the wall of the combustion chamber. In tactical weapons systems, the smoke produced by the firing of the motor enables effective countermeasures to be taken against both the missile and the launch site. Recent developments in polymer chemistry have produced hydroxy-terminated polybutadiene (HTPB) binders with which extremely high solids loading of ammonium perchlorate (AP) can be obtained. By obtaining 88 to 90% solid loadings of AP, high specific impulses can be obtained which are very useful in those applications in which smokeless propellants are required. Unfortunately, the high solids loading of AP with HTPB binder result in a propellant which is very susceptible to combustion instability and tends to aggravate the problem already existing as a result of the deletion of the aluminum fuel material. Accordingly, combustion instability is a major problem in advanced smokeless propellants. As has been pointed out above, combustion instability can be caused to occur in the more conventional state-of-the-art propellants merely by choosing to operate them at higher chamber pressures. Thus, while this invention is particularly useful in smokeless propellants, its utility should not be construed as being limited thereto. According to this invention, an additive in relatively small amounts has been found to extend the range of stable burning of composite propellant into regions which, for a particular composition, could not heretofore be obtained.